1. Field of the Invention
The present invention relates to a magnetoresistive random access memory and its write control method.
2. Related Art
The magnetoresistive random access memory (hereafter referred to as MRAM as well) is a memory device using a magnetic element using a magnetoresistive effect in a cell part which stores information. The magnetoresistive random access memory is drawing attention as a memory device of next generation having features of fast operation, a large capacity and non-volatility. The magnetoresistive effect is a phenomenon that the electric resistance changes according to the direction of magnetization in a ferromagnetic substance when a magnetic field is applied to the ferromagnetic substance. The ferromagnetic substance can be made to operate as a memory device by using the direction of magnetization in the ferromagnetic substance to record information and reading out information according to whether the electric resistance corresponding to the direction is large. In recent years, a magnetoresistive change rate (MR ratio) of 200% or more is obtained at room temperatures owing to the tunnel magnetoresistive effect (TMR effect) in a ferromagnetic tunnel junction including a sandwich structure having an insulation layer (a tunnel barrier layer) interposed between two ferromagnetic layers. Taking that opportunity, a MRAM using a magnetoresistive effect element (hereafter referred to as TMR element as well) having a magnetic tunnel junction (hereafter referred to as MTJ as well) is drawing expectation and attention.
When a TMR element is used as a magnetic element in a cell part of an MRAM, a magnetization pinned layer having a pinned magnetization direction is used as a reference layer which is one of two ferromagnetic layers having a tunnel barrier layer interposed between, and a magnetization free layer having a magnetization direction which can be inverted is used as a recording layer which is the other of the two ferromagnetic layers. Magnetization of the reference layer is pinned in one direction, and it is not inverted in a magnetic field of approximately 100 Oe. For example, in order to pin the magnetization direction in the reference layer, a method of providing an anti-ferromagnetic layer so as to be in contact with the reference layer and making magnetization inversion hard to occur by exchange coupling force is used. On the other hand, the recording layer is formed of a soft magnetic material, and it is inverted by a weak magnetic field in the order of 10 Oe. The tunnel barrier layer is formed of a very thin film having a thickness in the order of 1 nm.
If a voltage of approximately 0.5 V is applied from the recording layer toward the reference layer, then tunnel current flows. Its resistance value becomes low, when the recording layer is parallel in magnetization direction to the reference layer. A resistance value at this time is supposed to be R0. On the other hand, when the recording layer is anti-parallel in magnetization direction to the reference layer, the resistance value becomes high. A resistance value at this time is supposed to be R1. A function as a memory is provided by using a resistance value between the high resistance state and the low resistance state. The ratio of R0 to R1 is called MR ratio (Magnet Resistance Ratio), and it is found from the relation MR=(R1−R0)/R0. Although the value of the MR ratio differs depending upon the material included in the TMR element, it assumes a value in the range of several 10% to approximately 200%. A margin RM for readout is found from the value of the MR ratio and standard deviations δR0 and δR1 respectively of R0 and R1 according to the relation RM=MR/(δR0+δR1). As the MR ratio becomes larger and as resistance variations δR0 and δR1 become smaller, the readout margin RM becomes large.
In an MRAM having such TMR elements as storage elements of memory cells, bit lines and word lines are arranged across a TMR element from each other so as to nearly cross each other at right angles. The TMR element is provided in a crossing region of these writing wiring pieces. An induced magnetic field is generated by letting currents flow through these writing wiring pieces. Since two writing wiring pieces cross each other at right angles, induced magnetic fields generated when current are let flow cross each other. Writing is conducted by inverting the magnetization of the recording layer in the TMR element by the use of a current magnetic field generated from the two wiring pieces. For example, arrangement is conducted so as to typically cause a magnetic field generated by a current flowing through a bit line to be directed in a long side direction of the TMR element, i.e., in an axis of easy magnetization. As a result, the direction of magnetization in the recording layer after writing is determined. It is supposed that the magnetization after writing in the recording layer becomes anti-parallel, i.e., it is brought into the high resistance state (state of “1”) when a current is let flow through the bit line in a certain direction. If a current is let flow through the bit line in the opposite direction, the magnetization after writing in the recording layer becomes parallel, i.e., it is brought into the low resistance state (state of “0”). The magnetic field induced by a current flowing through the word line assists the inversion, but it does not determine the value written in.
In this way, the MRAM is set so as not to cause inversion when a current is let flow through either the bit line or the word line and so as to cause inversion only when a current is let flow through each of the bit line and the word line (see, for example, JP-A 2003-331574 (KOKAI)). As a result, two-axis selection becomes possible when the TMR elements are arranged in an array form.
Thus, magnetization inversion is conducted by letting currents flow through both the bit line and the word line. A TMR element in which a current is flowing through either the bit line or the word line is in a half-selection state. The current value is selected so as not to cause inversion in the TMR element in the half-selection state.
When a write current is let flow in order to shift the TMR element from the low resistance state to the high resistance state, the resistance value is anticipated to change from R0 to R1. On rare occasions, however, the resistance value becomes a middle value between R0 and R1. The middle value is denoted by R2. If the value of R2 is smaller than a value anticipated from the normal distribution of R1, this bit is brought into an intermediate state which is neither the low resistance state nor the high resistance state, resulting in false writing.
Also when writing is conducted so as to shift from the high resistance state to the low resistance state, the intermediate state is brought about in some cases.